Harvesting waste-heat energy of shell and tube heat exchanger used for desalination purposes using thermoelectric power generation modules: Numerical studies

This research introduces design and layout of a shell-and-tube heat exchanger (STHX) capable of producing electrical energy through a thermoelectric generator (TEG). The STHX is used simultaneously to enhance the efficiency of a desalinating-water unit through pre-warming of the sea water at 25 degrees C using waste heat from thermal power plants at 300 degrees C. Simulation using ANSYS FLUENT with realizable k-epsilon model is first considered to predict the thermo-hydraulic performance of STHX with considerable accuracy comparing with the well-known Bell-Delaware method. For fresh/salty cold water, the results indicate that the values of the outlet temperature of both water and air are increasing with increasing capacity ratio, while heat transfer rate, number of transfer unit (NTU), and heat exchanger effectiveness decrease. The results recommend the use of STHX with high water flow rate to obtain high heat transfer rate and effectiveness. Increasing the salinity (S) of salty water at the same mass flow rate causes increase of the outlet water temperature, heat transfer rate, NTU, and effectiveness but the air outlet temperature decreases, and the rate of change becomes high with the salinity value. The pumping power needed increases for both air and fresh/salty water with increasing mass flow rate, while it decreases with increasing the salinity of salty water. In the second part, TEG has been implemented in the STHX and simulated. The maximum efficiency of TEG is found to be about 5.5% at temperature difference of 375 K, and this means that the major deficiency of TEG is its lower efficiency. The voltage, the current, and the power increase with decreasing increment under the same temperature difference. The Joule heat produced in the TEG legs might change as the electrical current changes quickly. The enhanced Joule heat at a higher electrical current raises the temperature of cooling water.